The aim of this experiment was to determine plant responses to Coccus hesperidum L. infestation on two host plants. Groups of five lemon and five fern plants were colonised by various numbers of mobile C. hesperidum instar nymphs. After 6 months, all scale insect individuals were counted on each plant. According to the insect density, the plants were divided into a five-degree series. In all density classes of host plants tested, the infestation of scale insects decreased the chlorophyll and carotenoid content as well as the value of three indicators of photosynthetic activity. The strongest decrease in the analysed pigments was observed for the smallest abundance of insects (first-class density) in lemon leaves and in second-class density in fern leaves. The strongest reactions of the chlorophyll fluorescence indicators were observed in density classes III and IV in the fern leaves and density classes IV or V in the lemon leaves. The reactions depended on the specific properties of plants and abundance of insects feeding on them.

The study aimed at comparing the changes in the content of free amino acids, phenolic compounds and the activity of PAL and TAL caused by two piercing-sucking arthropods: the grape mealybug (Pseudococcus maritimus Ehrh.) and the two-spotted spider mite (Tetranychus urticae Koch) in the leaves of orchid and strawberry, respectively. The obtained results show that the amino acid content and the ratio of amino acids to phenolic compounds increased in both plant species infested by the mealybug and the mite. However, such response was weakly dependent on changes in activity of the analysed enzymes. The pest feeding affected accumulation of the phenolic compounds, since the induction of the PAL activity in mealybug-infested orchid leaves during the first 5 h of the experiment preceded the increase in phenolic compounds during the first week of insect feeding. Instead, the increased activity of TAL was accompanied by elevated levels of phenolic compounds in the leaves of strawberry infested by mites. Mechanisms of biochemical plant responses induced by infestation of the studied herbivorous arthropods are discussed.

Insect pests have major effects on agricultural production and food supply. Pest control in conventional crop management in orchards is mainly based on agrochemicals, which entails economic, health and environmental costs. Other approaches, such as biological methods or products based on biologically active substances and sex pheromones used for mating disruption, have faced some implementation challenges, particularly in relation to invasive insect species. The key for appropriate insect pest management is to identify the species and understand its biology and behaviour. Pest management systems should monitor, detect and inform fruit growers about changes in insect distribution, population ecology, possible damage level and economic loses. Comstockaspis perniciosa (San José scale—SJS) is a model example of a pest against which the method of integrated pest management should be adopted. This review presents a case study to support this statement.

Gall formation is induced by an insect, which changes normal plant development and results in the formation of a new organ, following distinct stages of metabolic and developmental alterations. Research on mechanisms of recognition and responses to biotic stress may help to understand the interactions between galling aphids and their host plants. In this study, Tetraneura ulmi L. (Hemiptera: Eriosomatinae) galls and Ulmus pumila L. (Rosales: Ulmaceae) leaves were used as a model. Concentrations of hydrogen peroxide (H₂O₂) and thiobarbituric acid reactive substances, electrolyte leakage, as well as the activity of ascorbate peroxidase, guaiacol peroxidase, and catalase (CAT) were determined in galls and two parts of galled leaves (with and without visible damage). Biochemical analyses were performed at three stages of gall development: initial, fully developed, and mature galls. A slight increment in H₂O₂ content with a strong enhancement of ascorbate peroxidase and CAT activities were observed in galls and galled leaves in the first stage. In subsequent stages of gall development, a progressing increase in H₂O₂ production and cell membrane damage was associated with declining antioxidant enzyme activities, especially in gall tissues. The stages of gall development are likely to be part of cell death triggered by aphid feeding. It seems that the gall is the result of a biochemical struggle between the host plant and the gall inducer.

This study aimed to determine the effects of plant species on the biological parameters of Tetranychus urticae Koch and the time of mite infestation on plant physiology in Ocimum basilicum L., Melissa officinalis L. and Salvia officinalis L. Mite infestation induced various levels of oxidative stress depending on plant species and the duration of infestation. Host plants affected T. urticae life table parameters. The low level of susceptibility was characteristic of S. officinalis, which appeared to be the least infected plant species and reduced mites demographic parameters. Infested leaves of S. officinalis contained elevated levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) compared to control. In addition, higher membrane lipid peroxidation and higher activity of guaiacol peroxidase (GPX) and lower activity of catalase (CAT) were recorded with a longer mite infestation. In contrast, O. basilicum appeared to be a suitable host on which T. urticae could develop and increase in number. In basil leaves, increasing levels of hydrogen peroxide and MDA with elevated GPX activity and strongly decreased catalase activity were recorded. Knowledge of the differences in mite susceptibility of the tested medicinal plants described in this study has the potential to be applied in breeding strategies and integrated T. urticae pest management in medicinal plant cultivations.

Pemphigus spyrothecae Pass. which is a member of the subfamily Pemphiginae is one of the gall-inducing aphids that occurs on poplar trees. Phloem feeding of a founding mother on leaf petiole results in the formation of a new organ, i.e., the spiral gall. This study documents aphid development inside the galls during the growing season and the effect of their feeding on leaf architecture and physiology of the host plant. In particular, leaf length, width, and area were measured, as well as hydrogen peroxide (H₂O₂) content, electrolyte leakage (E_L), malondialdehyde (MDA) concentration, and the activity of ascorbate (APX) and guaiacol peroxidase (GPX) were determined in galls and galled leaves. The presence of petiole galls significantly decreased the length, width, and leaf area. Aphid activity increased H₂O₂ concentration in galls and E_L from galls and leaf tissues, which was accompanied by a strong decrease in MDA content and both peroxidase activities, especially in gall tissues. It can be suggested that P. spyrothecae can manipulate physiological machinery of the host plant for its own benefit.